6" Pipe is actually 6-5/8" OD. If you want a 6" OD you will need to specify tube by OD and wall thickness.
Hope t his helps
G Austin
Are you looking for flue pipe? That can normally be gotten at a building supply store.
I assume this is for the spigot that the stovepipe hooks up to. I've always liked to hook the pipes up "backwards" that is with the small ends toward the stove. Then the condensate runs back into the stove instead of dripping out on the floor. If you agree then you would want 6" id. Most contractors that install gas piping in the street will have some that they have removed from time to time. Usually cheap.
Bill